1. Field of the Invention
The present invention relates to a base station and a mobile station to be used in a mobile communication system.
2. Description of the Background Art
First, the conventional mobile communication system will be briefly described for a case of a CDMA (Code Division Multiple Access) scheme. In the CDMA mobile communication scheme, in order to alleviate the so called Near/Far problem related to the variation of distances from the base station to the mobile stations, the base station carries out the transmission power control with respect to the mobile station in communication such that the radio wave from all the mobile stations can be received at a uniform receiving level at the base station. Here, all the mobile stations carry out the communications at the same frequency so that the communications of the mobile stations interfere with each other and the communication quality is determined by this interference. However, in order to increase the subscriber capacity, the transmission power from the base station is set to be the least possible power for guaranteeing such a quality that the radio wave for the particular mobile station does not interfere with the other mobile stations. See, E. Kudoh and T. Matsumoto: "Effect of Transmitter Power Control Imperfections on Capacity in DS/CDMA Cellular Mobile Radios", IEEE ICC '92, pp. 310.1.1-310.1.6, for further detail.
In the CDMA mobile communication scheme, the radio channels are formed by using the same frequency and the different spread codes, so that when the mobile station moves to another cell (zone), the radio channel can be switched by changing the spread code without changing the frequency. Also, when the mobile station is capable of executing a received baseband signal processing using a plurality of correlators, it is also possible to set up a plurality of radio channels with respect to a plurality of base stations by using different spread codes. By utilizing this function at a time of the radio channel switching, it is possible to carry out the simultaneous communications with the base station to move in and the base station to move out so as to prevent the breaking of the communication at a time of the radio channel switching, which contributes to the improvement of the communication quality and the service in general. This operation is known as the soft handover.
In the soft handover state, the mobile station is going to transmit the same data through a plurality of radio channels to different base stations simultaneously, so that the site diversity effect can be expected. By this diversity effect, the bit error rate characteristic can be improved as the number of branches M increases, as indicated in FIG. 1. See, M. Schwartz, et al.: "Communication Systems and Techniques", pp. 459-466, MaGraw-Hill, 1966 for the detailed information related to the indication of FIG. 1.
Thus, when the mobile station is in the soft handover state, the communication quality in each radio channel is degraded, but there are cases in which the sufficient characteristic can be obtained for the overall characteristic after the composition of a plurality of radio channels because of the site diversity effect.
However, in the conventional mobile communication scheme, when there are two base stations to communicate with, the target receiving level or the target interference ratio level to reach to both of the base stations at a time of the transmission power control is set to be identical as in a case of the communication through the single radio channel. Consequently, despite the fact that the overall quality after the composition is satisfying the threshold communication quality, the transmission power control at the same target level is carried out, so that the interference with the communications of the other mobile stations increases and the system capacity decreases.
In addition, in this soft handover, a number of transmission channels in communication between the base stations and the mobile stations increases by one radio channel, so that the transmission power must be increased for one transmission channel part. This implies that the interference with the communications of the other mobile stations is increased for one transmission channel part. For this reason, the interference with the communications of the other mobile stations increases and the system capacity decreases.
Furthermore, as the conditions for triggering the switching of the mobile station to the other base station, the threshold bit error rate and the threshold receiving level are used, but in the conventional mobile communication scheme, these conditions are maintained to be constant no matter how many base stations are to be connected. In other words, when a plurality of base stations are in communication, despite the fact that the overall quality after the composition is satisfying the threshold communication quality due to the site diversity effect, the radio channel switching is going to be carried out, so that the number of radio channel switching operations increases, and the control load on the base station and the mobile station increases.
Thus, the subscriber capacity of the system as a whole can be reduced when the target receiving level, the target interference ratio level, or the threshold communication quality for triggering the channel switching is set to be the same for both a case of communicating with only one base station and a case of communicating with a plurality of base stations as in the soft handover.